Types of Forces
Gravitational Force
 An attractive force between all objects that have mass.
 On Earth gravity is a downward force, always pulling objects
towards the center of Earth
Weight and Mass
 Mass is the amount of matter in an object and does not
change with location.
 Weight is the gravitational force on an object and changes
with location.
Friction
 Friction is a force that opposes the movement between two
surfaces in contact.
 EXAMPLE
 A book pushed across a table slows down because of friction.
Static Friction
 The force between two surfaces in contact that keeps them
from sliding when a force is applied.
 EXAMPLE
 A force is applied to a heavy box, but the box doesn’t move.
 The forces are balanced, the force pushing the box equals the
force of static friction pushing in the opposite direction.
Kinetic Friction
 Friction force that acts on objects that are moving
 EXAMPLE
 The frictional force that acts on the sliding book is called kinetic
friction.
Fluid Friction
 A force that opposes the motion of an object when an object
moves through a fluid
Air Resistance
 Fluid friction acting on an object moving through the air
Elastic Forces
 An elastic force occurs when a material is stretched or
compressed.
 EXAMPLE
 A diving board exerts an upward elastic force on the diver when it
is bent downward.
Elastic Force: Tension
 A pulling force exerted by an object when it is stretched,
such as a rubber band.
Elastic Force: Compression
 A pushing force exerted by a material when it is squeezed
or compressed.
 The size of the compression force exerted by a material is
equal to the size of the force that compresses the material.
Normal Force
 The force exerted by an object that is
perpendicular to the surface of the
object.
 Weight acts in a direction opposite of a
normal force
 The support force exerted upon an
object that is in contact with another
stable object
 EXAMPLE
 The cup is exerting a downward force on the
table, caused by gravity.
 The table is exerting an upward normal force
on the cup, caused by compression.
Normal Force
Gravity
Forces in the Vertical Direction
 Upward normal force balances the downward force of gravity
on an object that is not moving vertically.
Centripetal Force
 A force that makes an object follow a curved path
 As an object moves in a circle, it is constantly changing its
direction.
 Because of this direction change, you can be certain that an
object undergoing circular motion is accelerating (even if it is
moving at constant speed)
 Without a centripetal force, an object in motion continues
along a straight-line path.
Force Diagrams
Normal Force
Friction
Applied PUSHING
Force
Gravity
20 N
5N
15 N
3N
You try
 With your group, create a situation in which normal force,
gravity, and friction affect the overall motion of an object. Draw
a force diagram that represents the situation. Be ready to
present the situation to the class.
 EXAMPLE
 A person is trying to move their book across the table. Gravity
causes a downward force on the book, normal force acts in an
upward direction, a force is applied in one direction and friction acts
in the opposite direction of the applied force.
 YOU MAY NOT USE THIS EXAMPLE!!!
Force Diagram WKSHcomplete it
Addressing
Misconceptions
Sustained Forces
 Objects do NOT require a sustained force to stay in motion.
 FRICTION causes objects to slow down.
 EXAMPLE: A car’s engine propels it forward. Friction
between the tires and the road, and between the body of the
car and surrounding air, cause the car to slow down.
Newton’s First Law of Motion
If there is
will
NO FORCE acting on an object, the object
not CHANGE
it’s motion.
Newton’s first law: An object in motion STAYS in motion
UNLESS a force acts on it?
What force causes a ball rolling on the
ground to slow down and eventually
come to a stop?